1. Field of the Invention
The present invention relates to improvements to the supply circuit of an electrical decelerator for heavy vehicles such as road transport vehicles or motor coaches.
2. Description of the Prior Art
"TELMA" electrical decelerators installed in many commercial vehicles of medium and large tonnage, touring and grand touring motor coaches, are well known to users. A documented article on these decelerators has been published in the technical review "Le poids lourd" ["The Truck"] October 1970, pages 79 to 88.
The Telma electrical equipment comprises a box of relay switches connected by a bundle of heavy cables to the decelerator and by a bundle of four fine wires to a control device. This box contains four breaking relays corresponding to four positions which are obtained by the successive excitation of four identical independent circuits mounted on the decelerator. Depending on the use of the vehicle, the control can be manual, mechanical or pneumatic. The braking is controlled by a lever mounted on the steering column like that for controlling the lights. The lever has five-positions with an off-position and four steps of increasing braking power. The control system, which is thus a fiveposition switch, can be actuated either manually or by the main brake pedal. It activates in succession four power relays which each supply two windings of the decelerator.
The major drawback of the existing system is that its actuation is not progressive but leads quite to the contrary to sharp variations in the braking torque. Measurements on a (TELMA) decelerator of the FOCAL 115 type, 28 volts at 1000 RPM, have yielded the following braking torque date.
1st contact: 22 mda N
2nd contact: 52 mda N
3rd contact: 81 mda N
4th contact: 118 mda N
The existing electrical decelerator has no regulation, since this function must be provided by the driver. This fact makes it difficult to couple the decelerator with the main brake on a motor coach and makes driving particularly tiresome.
On the other hand, this actuation following driver action risks the application of too great braking forces and can lead to loss of traction at the rear (AR) at low braking levels (empty vehicle, slippery road). Since the decelerator acts only on two wheels. This actuation is always too great in the case of foot command (successive actuation of four relays, then pneumatic braking); the pneumatic braking being insufficient in itself to exploit the full traction of the AR axle.
In view of the fact that road-transport vehicles are subject to norms concerning equitraction, the control of the decelerator by the brake pedal becomes impossible. The drive must therefore make use of a supplementary device.